With diversification of foods, foods having various shapes, physical properties and textures have been required. Particularly, intense interest has recently been shown towards melt in mouth and texture as important physical properties for the purpose of designing foods. Also in the fields related to deglutition and care toward which intense interest has recently been shown, texture has been studied as important physical properties.
In the case of designing processed foods, utilization of a gelling agent is important so as to improve texture and physical properties, and it is possible to develop various products according to how to use.
For the purpose of altering physical properties of foods, various gelling agents have hitherto been added to food materials in the case of preparing foods.
In food processing, natural macromolecules such as agar, gelatin, gellan gum, xanthan gum, locust bean gum, carrageenan, pectin, sodium alginate, Tamarind seed gum, psyllium seed gum, microcrystalline cellulose, curdlan, and starch; or synthetic macromolecules such as carboxymethyl cellulose (CMC) or methyl cellulose are commonly used as gelling agents.
In the case of using these gelling agents, gelling agents may be sometimes used alone, however, in order to form gels having more various characteristics, for example, use of two or more kinds of gelling agents such as native gellan gum and guar gum in combination is studied and utilized (Patent Document 1).
However, there are few combinations which can synergistically change the gel strength of foods. Even if it is possible to synergistically change the gel strength, the gel obtained thereby does not have nice physical properties. Mixing of two or more kinds of gelling agents is a defect due to being complicated and that many materials are very expensive.
Furthermore, there is such a restriction on use in food processing that, for example, a gelatin is inferior in resistance to an acid and an alkali, and also an agar is inferior in resistance to an acid.
Starches have successfully produced various physical properties by adding not only raw starches but also a processed starch obtained by chemically modifying starches (also referred to as a chemically modified starch) such as starch acetate and monostarch phosphate as a gelling agent to food materials. For example, Patent Documents 2, 3 and 4 indicate examples in which a crosslinked starch is utilized in a white table bread, confectioneries or noodles. However, in the case where a crosslinked starch having a high crosslinking degree is added to a food, the hardness and the viscosity of a gel can be enhanced, but there is such a drawback that a final product has powdery texture and also is inferior in flavor. Also, in the case where a starch having a low crosslinking degree is added to a food, since a large amount of the starch needs to be used so as to obtain the desired hardness, the obtained food has increased powdery texture, thus causing deterioration of quality of a final product. Therefore, there is a limit on the use amount of the starch having a low crosslinking degree. In addition, processing of the starch utilizing a chemical reaction also has such problems that there is a strict legal restriction on a processing method and a processing degree so as to secure safety, and that it is not necessarily suited to needs of consumers who require security and safety.
For the purpose of deigning these processed foods, it is urgently necessary to develop a processing technique to obtain a processed starch which exhibits various physical properties and has high safety.
As a result of intensive studies, we have found that a food with rich elasticity, crispy sensation and the like can be prepared by adopting the steps of treating starch granules with a starch hydrolase or a glycosyltransferase in advance; then mixing the resultant with a food material and water; and heating the mixture.
A starch is a material utilized for various purposes and the most important function thereof is the thickening function and the gel forming function. Particularly in the food industry, the thickening function and the gel forming function of the starch are widely utilized for forming the shape, physical properties and texture of a food. The structure of a starch delicately varies depending on plant from which the starch derived (for example, corn, potato, wheat, and cassava). As a result, the thickening function and gel forming function also vary depending on the plant from which the starch derived. Therefore, those skilled in the art have been selected a native starch to be used for a long time depending on the purpose. For example, a wheat starch has often been used in a fish paste product for a long time. The reason is that the wheat starch is excellent in gel forming function. For example, a cassava starch is commonly utilized in a food which has high transparency and requires sticky texture. However, with the advancement of characteristics required in the current food industry, it becomes impossible to cope with the advancement only by changing a native starch to be used. Therefore, there arises the need to alter the thickening function or the gel forming function of a starch.
Means which are used most commonly to alter the thickening function or the gel forming function of a starch is a chemical modification of a starch. Above all, techniques of applying a chemical treatment, such as a technique of introducing a new crosslinking point between starch molecules using a suitable chemical crosslinking agent and a technique of introducing a suitable functional group have widely been utilized so as to remarkably alter the thickening function or the gel forming function. However, a starch subjected to such a chemical treatment has been specified as a food additive from October, 2008 in Japan, and thus restricted by law. Therefore, there has been required a technique in which the thickening function or the gel forming function of a starch is altered without a chemical treatment.
The technique of altering a starch without a chemical treatment includes a technique of an enzymatic treatment of a starch. Since an enzyme commonly acts on a substrate dissolved in water, an enzymatic treatment is usually carried out after completely dissolving a starch in water. A hydrolytic enzyme or a glycosyltransferase is allowed to act on a starch dissolved in water to cleave the starch, thereby producing molecules having a lower molecular weight such as dextrin, starch syrup, maltooligosaccharide, maltose, and glucose. However, in the enzymatic treatment with a hydrolytic enzyme or a glycosyltransferase, a starch molecule is cleaved to form low-molecular weight molecules. Therefore, it has been commonly considered that the thickening function and the gel forming function of the obtained molecule deteriorate as compared with the thickening function and the gel forming function of the starch, or are lost.
Also, Patent Document 5 discloses, as a method of altering physical properties of a starch, a technique in which an enzyme is allowed to act on a starch in the form of starch granules in water without dissolving them in water. Patent Document 5 discloses that although a starch has conventionally been dissolved in water before an enzymatic treatment in the case of subjecting the starch to the enzymatic treatment, it is not necessarily required to dissolve the starch in water before the enzymatic treatment, and it is possible to subject starch granules, which are not dissolved in water but suspended in water, to the enzymatic treatment. Specifically, it is disclosed that a hydrolytic enzyme such as α-amylase or glucoamylase can act on starch granules, which are not dissolved in water but suspended in water, and thus a reducing sugar can be produced. Patent Document 5 also discloses as a result of this that the viscosity of the starch subjected to the enzymatic treatment is lower than that of the starch which is not subjected to the enzymatic treatment. However, Patent Document 5 neither suggests nor discloses that a starch having improved thickening function or gel forming function as compared with the starch, which is not subjected to the enzymatic treatment, is obtained by allowing a hydrolytic enzyme or a glycosyltransferase to act on starch granules.
Patent Documents 6 to 10 also disclose a technique of allowing a hydrolytic enzyme to act on insoluble starch granules. These inventions disclose a technique in which the action of a hydrolytic enzyme on starch granules opens pores on the surfaces of starch granules to make porous starch granules, and the porous starch granules are utilized as a powdered base material or a porous carrier. However, Patent Documents 6 to 10 neither suggests nor discloses that a starch having improved thickening function and gel forming function is obtained by allowing a hydrolytic enzyme or a glycosyltransferase to act on starch granules. An object of the present invention is not to open pores on the surfaces of enzyme-treated starch granules, and there is not any relationship between an improvement in thickening function and gel forming function, and whether or not pores are opened on the surfaces of enzyme-treated starch granules. If a heated food is produced using the enzyme-treated starch of the present invention, the enzyme-treated starch forms a hard gel in the heated food. The enzyme-treated starch of the present invention is usable in the heated food. On the other hand, in the prior art, it is important that pores are present on the surfaces of starch granules. If starch granules after subjected to the enzymatic treatment and water are mixed and then heated, starch granules are collapsed and pore-opened states thereof are lost. Therefore, those skilled in the art did not consider to use a pore-opened starch of the prior art in the heated food. In the present invention, it is possible to adjust the hardness of a gel to be formed using an enzyme-treated starch by adjusting the degree of the enzymatic treatment. The hardness of the gel exerts an influence on texture, chewiness, and the like of the food. Therefore, use of the method of the present invention can exert an influence on texture of the food. As described above, the enzyme-treated starch granules of the prior art and the enzyme-treated starch granules used in the present application quite differ in application and usage.
As described above, it was conventionally impossible to provide a starch excellent in thickening function or gel forming function without utilizing a chemical modification of a starch.
Also, in the prior art, no attention was paid at all whether or not an enzyme has characteristics capable of improving a gel forming ability of a starch. It was not also found at all whether or not industrial advantages are exerted by characteristics of an enzyme capable of improving a gel forming ability of a starch.